<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>

  <style>
    body{
      /*css三大特征: 继承 层叠 优先级*/
    }
    #d1{
      color: red;
    }
    div{
      /*!important 作用是提升优先级*/
      color: blue !important;
    }
  </style>

</head>
<body>

<div id="d1">
  <p>这是个p标签</p>
  <span>这个是div里面的span</span>
  <a href="">超链接</a>
</div>
<span>这个是div外面的span</span>

</body>
</html>